The natural multisystemic disease syndromes of inbred and crossbred New Zealand mice are well recognized laboratory animal models of human diseases of unknown etiology: systemic lupus erythematosus, autoimmune hemolytic anemia, and lymphoma/leukemia. We have shown that the natural history of the murine disease is related to the expression of an endogenous type C virus and, particularly, the viral envelope glycoprotein antigen gp70 and to pathogenetic antibody responses to self antigens and endogenous viral protein antigens. A continuing objective is to elucidate the genetic, viral, and immunological determinants of disease in the animal models and to apply the insight gained to the study of the cause, nature and control of corresponding human diseases. To this end, we have found that an antigen related to the major core protein of mammalian (including primate) type C viruses is located in human tissues in the disease, systemic lupus erythematosus. The nature and significance of this antigen is under investigation. BIBLIOGRAPHIC REFERENCES: The occurrence and frequency of type C virus-like particles in placentas from patients with systemic lupus erythematosus and from normal subjects. Imamura, M., Phillips, P.E., and Mellors, R.C. Am. J. Pathol. 83:383-394, 1976. Murine type C viral envelope glycoprotein gp69/71 and lupus-like glomerulonephritis of New Zealand mice: an immunoperoxidase study. Imamura, M., Mellors, R.C., Strand, M., and August, J.T. Am. J. Pathol. 86:375-386, 1977.